Semiconductor device and manufacturing method of semiconductor device

ABSTRACT

A solder joint layer has a structure in which plural fine-grained second crystal sections ( 22 ) precipitate at crystal grain boundaries between first crystal sections ( 21 ) dispersed in a matrix. The first crystal sections ( 21 ) are Sn crystal grains containing tin and antimony in a predetermined proportion. The second crystal sections ( 22 ) are made up of a first portion containing a predetermined proportion of Ag atoms with respect to Sn atoms, or a second portion containing a predetermined proportion of Cu atoms with respect to Sn atoms, or both. The solder joint layer may have third crystal sections ( 23 ) which are crystal grains that contain a predetermined proportion of Sb atoms with respect to Sn atoms. As a result, solder joining is enabled at a low melting point, and a highly reliable solder joint layer having a substantially uniform metal structure can be formed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The embodiments discussed herein are related to a semiconductor deviceand a manufacturing method of a semiconductor device.

2. Description of the Related Art

Conventionally known semiconductor devices include devices that have apackage structure in which a semiconductor chip is joined to a circuitpattern that is provided on an insulating substrate, and soldermaterials that enable joining at a comparatively low temperature areused as joining materials for joining the semiconductor chip and thecircuit pattern. As such solder materials, solder having tin (Sn) as amain component, for instance tin-silver (Sn—Ag)-based solder materialsand highly reliable tin-antimony (Sn—Sb)-based solder materials thatenable joining at a low melting point, is used. The state of aSn—Ag-based solder material after solder joining and the state of aSn—Sb-based solder material will be discussed next.

The melting point of a solder having Sn as a main component ranges fromabout 200° C. to 300° C. A solder joint layer that utilizes solderhaving Sn as a main component exhibits a structure having Sn crystalgrains dispersed therein. In a solder joint layer that utilizes a Sn100% solder material, Sn crystal grains undergo coarsening at hightemperature; moreover, changes in temperature cause the solder jointlayer to be subject to stress derived from differences in thecoefficient of linear expansion with respect to non-joining materials.As a result, a problem arises in that grain boundary cracks occur at thecrystal grain boundaries between Sn crystal grains, and these grainboundary cracks progress to crystal grain boundaries between adjacent Sncrystal grains. Known solder materials in which such progress of grainboundary cracks is prevented include Sn—Ag-based solder materials andSn—Sb-based solder materials.

FIG. 7 is an explanatory diagram illustrating schematically the state ofa solder joint layer by a conventional Sn—Ag-based solder material. InFIG. 7, (a) illustrates an initial state (before application of athermal load, for instance from power cycling) of a solder joint layerby a conventional Sn—Ag-based solder material (hereafter referred to asSn—Ag-based solder joint layer). The Sn—Ag-based solder material is aprecipitation-strengthened solder material. As illustrated in (a) ofFIG. 7, Ag forms virtually no solid solution in Sn crystal grains insolder joint layers that utilize a conventional Sn—Ag-based soldermaterial; accordingly, Ag yields a fine-grained hard Ag₃Sn compound 122,and precipitates at crystal grain boundaries between Sn crystal grains121 that are dispersed as a matrix. The crystal grain boundaries betweenSn crystal grains 121 are strengthened as a result and the crystals donot deform readily. Consequently, the grain boundary cracks progressless readily than is the case in solder joint layers of simple Sncrystal grains.

FIG. 8 is an explanatory diagram illustrating schematically the state ofa solder joint layer by a conventional Sn—Sb-based solder material. InFIG. 8, (a) illustrates the initial state of a solder joint layer by aconventional Sn—Sb-based solder material (hereafter referred to asSn—Sb-based solder joint layer).

The Sn—Sb-based solder material is a solder material of solid-solutionstrengthened type. As illustrated in (a) of FIG. 8, Sb forms a solidsolution up to about 8.5 wt % (8.3 atom percent (at %)), such that theSn crystal grains 131 overall are strengthened, in a solder joint layerthat utilizes a conventional Sn—Sb-based solder material in Sn crystalgrains 131.

Coarsening of the Sn crystal grains 131 arising on account of thermalload in repeated cycles of heat generation and heat dissipation duringthe operation of a semiconductor device can be suppressed throughstrengthening of the Sn crystal grains 131 by the Sb in solid solution.Further, Sb in excess of the solid solution limit precipitates partly inthe form of a stiff SnSb compound 132 along with part of the Sn in theSn crystal grains 131. As a result, the crystals deform less readily,and intra-grain cracks progress less readily.

As such a solder material that includes Sn, Ag and Sb, a solder materialhas been proposed where, in order to enhance the thermal fatiguecharacteristic of joints, the content of oxygen (O₂) as an unavoidableimpurity is set to be equal to or smaller than 5 ppm, and the averagegrain size to be equal to or smaller than 3 μm, in an Sn alloy soldercontaining one or two of Ag: 1% to 30% and Sb: 0.5% to 25%, with thebalance made up substantially of Sn and unavoidable impurities (see, forinstance, Japanese Patent Application Publication No. S61-269998).

As yet another solder material, a solder material has been proposed thatincludes 5 wt % to 15 wt % of Sb and 2 wt % to 15 wt % of Ag, with thebalance made up substantially of Sn, excluding unavoidable impurities,and where the surface roughness of the solder material is Ra=10 μm orsmaller (see, for instance, Japanese Patent Application Publication No.H07-284983).

As yet another solder material, a solder material has been proposed thatis a composite solder material containing a powder in a solder material,where the solder material includes 5 wt % to 15 wt % of Sb and 2 wt % to15 wt % of Ag, with the balance made up substantially of Sn, excludingunavoidable impurities (see, for instance, Japanese Patent ApplicationPublication No. H08-001372).

As yet another solder material, a solder material has been proposed thatis made up of an alloy including 25 wt % to 40 wt % of Ag, 24 wt % to 43wt % of Sb, and the balance Sn, where the melting temperature of thesolder material is set to at least 250° C. or higher (see, for instance,Japanese Patent Application Publication No. 2003-290975).

As yet another solder material, a solder material has been proposed thatincludes, in mass %, Ag: 0.9% to 10.0%, Al: 0.01% to 0.50%, Sb: 0.04% to3.00%, such that the ratio Al/Sb satisfies a relationship of being equalto or smaller than 0.25 (excluding 0), the balance being Sn andunavoidable impurities, where the solder material joins a member havingoxide or an oxidized surface (see, for instance, Japanese PatentApplication Publication No. 2011-005545).

As yet another solder material, a solder material has been proposed thatincludes 0.05 mass % to 2.0 mass % of Ag, 1.0 mass % or less of copper(Cu), 3.0 mass % or less of Sb, 2.0 mass % or less of bismuth (Bi), 4.0mass % or less of indium (In), 0.2 mass % or less of nickel (Ni), 0.1mass % or less of germanium (Ge), 0.5 mass % or less of cobalt (Co)(where, none of Cu, Sb, Bi, In, Ni, Ge, and Co is 0 mass %), and tin asthe balance (see, for instance, Japanese Patent No. 4787384).

As yet another solder material, a solder material has been proposed thathas a SnSbAgCu substance as a main component, and the composition of thesolder material is 42 wt %<Sb/(Sn+Sb)≤48 wt %, 5 wt %≤Ag<20 wt %, 3 wt%≤Cu<10 wt %, and 5 wt %≤Ag+Cu≤25 wt %, the remainder being unavoidableimpurity elements (see, for instance, Japanese Patent No. 4609296).

As yet another solder material, a solder material has been proposed thatincludes 12 mass % to 16 mass % of Sb, 0.01 mass % to 2 mass % of Ag and0.1 mass % to 1.5 mass % of Cu, and further includes 0.001 mass % to 0.1mass % of silicon (Si), and 0.001 mass % to 0.05 mass % of B, withrespect to a high-temperature solder material as a whole, the balancebeing Sn and unavoidable impurities (see, for instance, Japanese PatentNo. 4471825).

As yet another solder material, a solder material has been proposed thathas, as a main component, a SnSbAgCu substance having a solidustemperature of 225° C., where the constituent ratio of the alloy is 10wt % to 35 wt % of Ag and Cu, and the weight ratio of Sb/(Sn+Sb) rangesfrom 0.23 to 0.38 (see, for instance, Japanese Patent ApplicationPublication No. 2005-340268).

As yet another solder material, a solder material has been proposed thathas, as a base, an Sn—In—Ag solder alloy including 88 mass % to 98.5mass % of Sn, 1 mass % to 10 mass % of In, 0.5 mass % to 3.5 mass % ofAg and 0 mass % to 1 mass % of Cu, the Sn—In—Ag solder alloy being dopedwith a crystallization improver that suppresses growth of anintermetallic phase in the solidified solder (see, for instance,Japanese Translation of PCT Application No. 2010-505625).

As yet another solder material, a solder material has been proposed thatincludes Ag: 2 mass % to 3 mass %, Cu: 0.3 mass % to 1.5 mass %, Bi:0.05 mass % to 1.5 mass % and Sb: 0.2 mass % to 1.5 mass %, where thetotal content of Ag, Cu, Sb and Bi is 5mass% or less, and the balanceincludes Sn and unavoidable impurities, and the surface properties afterreflow are smooth (see, for instance, Japanese Patent ApplicationPublication No. 2002-018590). Reflow is a soldering method where a layerof a solder paste (paste obtained by adding a flux to a solder powder,and adjusted to appropriate viscosity) is formed on a joining material,a component is placed on the solder paste, and thereafter heat isapplied to melt the solder.

As yet another solder material, a solder material has been proposed thatincludes 1 mass % to 3 mass % of Ag, 0.5 mass % to 1.0 mass % of Cu, 0.5mass % to 3.0 mass % of Bi, 0.5 mass % to 3.0 mass % of In, 0.01 mass %to 0.03 wt % of Ge or 0.01 mass % to 0.1 mass% of selenium (Se), and thebalance being Sn (see, for instance, Japanese Patent ApplicationPublication No. 2001-334385).

As yet another solder material, a solder material has been proposed thatincludes 15.0% to 30.0% of Bi and 1.0% to 3.0% of silver, and dependingon the circumstances, optionally 0% to 2.0% of copper, and 0% to 4.0% ofSb and incidental impurities, and the balance being Sn (see, forinstance, Japanese Translation of PCT Application No. 2001-520585).

As yet another solder material, a solder material has been proposed thatis a Sn—Sb—Ag—Cu quaternary alloy that, with respect to the total,contains proportions of 1.0 wt % to 3.0 wt % of Sb, 1.0 wt % or more butless than 2.0 wt % of Ag, and 1.0 wt % is or less of Cu, with Sn as thebalance (see, for instance, Japanese Patent Application Publication No.H11-291083).

As yet another solder material, a solder material has been proposed thatcontains 3.0 wt % or less of Sb (excluding zero as the range lowerlimit), 3.5 wt % or less of silver (excluding zero as the range lowerlimit), 1.0 wt % or less of Ni (excluding zero as the range lowerlimit), 0.2 wt % or less of phosphorus (P) (excluding zero as the rangelower limit), and the balance includes Sn and unavoidable impurities(see, for instance, Japanese Patent No. 3353662).

As yet another solder material, a solder material has been proposed thatcontains 2.5 wt % to 3.5 wt % of Sb, 1.0 wt % to 3.5 wt % of Ag and 1.0wt % or less (excluding zero as the range lower limit) of Ni, and thebalance includes Sn and unavoidable impurities (see, for instance,Japanese Patent No. 3353640).

As yet another solder material, a solder material has been proposed thatincludes 0.5 wt % to 3.5 wt % of Ag, 3.0 wt % to 5.0 wt % of Bi, 0.5 wt% to 2.0 wt % of Cu, 0.5 wt % to 2.0 wt % of Sb and the balance beingSn, the solder material being in any one form from among rod-like,wire-like, preform-like or flux-cored solder (see, for instance,Japanese Patent No. 3673021).

As yet another solder material, a solder material has been proposed thatincludes 0.8 wt % to 5 wt % of Ag, and In and Bi each in an amount of0.1 wt % or greater, with the total amount of both being 17 wt % orless, and a balance including Sn and unavoidable impurities, and wherethe solder material has further added thereto 0.1 wt % to 10 wt % of Sb(see, for instance, Japanese Patent Application Publication No.H09-070687).

As yet another solder material, a solder material has been proposed thatcontains 61 wt % to 69 wt % of Sn, 8 wt % to 11 wt % of Sb, and 23 wt %to 28 wt % of Ag (see, for instance, U.S. Pat. No. 4,170,472).

As yet another solder material, a solder material has been proposed thatincludes 93 wt % to 98 wt % of Sn, 1.5 wt % to 3.5 wt % of Ag, 0.2 wt %to 2.0 wt % of Cu and 0.2 wt % to 2.0 wt % of Sb, and that has a meltingpoint ranging from 210° C. to 215° C. (see, for instance, U.S. Pat. No.5,352,407).

As yet another solder material, a solder material has been proposed thatincludes 90.3 wt % to 99.2 wt % of Sn, 0.5 wt % to 3.5 wt % of Ag, 0.1wt % to 2.8 wt % of Cu and 0.2 wt % to 2.0 wt % of Sb, and that has amelting point ranging from 210° C. to 216° C. (see, for instance, U.S.Pat. No. 5,405,577).

As yet another solder material, a solder material has been proposed thatincludes at least 90 wt % of Sn, and an effective amount of Ag and Bi,and that includes optionally Sb, or Sb and Cu (see, for instance, U.S.Pat. No. 5,393,489).

As yet another solder material, a solder material has been proposed thatincludes 0.5 wt % to 4.0 wt % of Sb, 0.5 wt % to 4.0 wt % of zinc (Zn),0.5 wt % to 2.0 wt % of Ag and 90.0 wt % to 98.5 wt % of Sn (see, forinstance, U.S. Pat. No. 4,670,217).

As yet another solder material, a solder material has been proposed thatis a solder paste including a metal component made up of a first metalpowder and a second metal powder having a melting point higher than thatof the first metal powder, where the first metal is Sn alone, or analloy including Sn and at least one element selected from the groupincluding Cu, Ni, Ag, gold (Au), Sb, Zn, Bi, In, Ge, Co, manganese (Mn),iron (Fe), chromium (Cr), magnesium (Mg), palladium (Pd), Si, strontium(Sr), tellurium (Te) and P (see, for instance, WO 2011/027659).

Semiconductor devices are subject to thermal load upon repeated heatgeneration and heat dissipation (power cycling) during operation of thedevice, and also by thermal load from heat cycles such as changes inenvironmental temperature or the like (heating and cooling).Deterioration of solder joint layers on account of thermal load derivedfrom such power cycling and the like has conventionally been a problemin semiconductor devices. The life of solder joint layers is onedetermining factor of the life of semiconductor devices, and it isaccordingly necessary to prolong the life of solder joint layers. Inorder to reduce the size of the semiconductor device as a whole and thesize of heat sinks, it is necessary to operate during high-temperatureheat generation by the semiconductor device (for instance, 175° C. orhigher) and, in particular, to secure the power cycling reliability ofpower semiconductors at this temperature. Further, semiconductor devicesinstalled in automobiles and semiconductor devices in new energyapplications must be long-lived. Consequently, solder materials arerequired that enable solder joining at a low melting point and thatallow forming a solder joint layer of high reliably towards powercycling and the like. Power cycling reliability includes variouscharacteristics of the semiconductor device when the latter is operatedand is accordingly subject to a load of a predetermined temperaturecycle.

For instance, a Sn3.5Ag solder material (solder material including 96.5wt % of Sn and 3.5 wt % of Ag) ordinarily used as a conventionalSn—Ag-based solder material described above allows for solder joining ata low melting point (for instance, about 220° C.), but is problematic interms of reliability during high-temperature operation. When the Agcontent in a Sn—Ag-based solder joint layer is increased, as in JapanesePatent Application Publication Nos. S61-269998, H07-284983, H08-001372,2003-290975, and 2011-005545, material costs increase as well (forinstance, a solder cost increase of about 20% for a 1% increase in Agcontent), and the melting point becomes higher (for instance, a meltingpoint of about 300° C. in a Sn10Ag solder material (solder materialincluding 90.0 wt % of Sn and 10.0 wt % of Ag)). It is accordinglyimpractical to increase the Ag content in Sn—Ag-based solder jointlayers.

The following problems arise in conventional Sn—Ag-based solder jointlayers due to thermal load in power cycling. In FIG. 7, (b) illustratesthe state of conventional Sn—Ag-based solder joint layer at the time ofa power cycling reliability test (state resulting from being subject toa thermal load from power cycling). In a conventional Sn—Ag-based solderjoint layer, Sn crystal grains 121 undergo coarsening on account ofthermal load from power cycling, and the Ag₃Sn compound 122 undergoesaggregation and coarsening to a grain size of about 5 μm, as illustratedin (b) of FIG. 7.

Consequently, the crystal grain boundaries between Sn crystal grains 121are no longer strengthened by the Ag₃Sn compound 122 and, as a result, agrain boundary crack 123 arises at crystal grain boundaries between Sncrystal grains 121. This grain boundary crack 123 progresses to thecrystal grain boundaries between adjacent Sn crystal grains 121.

The above-described conventional Sn—Sb-based solder material isproblematic in that although the material becomes more reliable as thecontent of Sb included in the Sn—Sb-based solder material increases, themelting point of the material rises as the content of Sb increases. Forinstance, the melting point of a Sn13Sb solder material (solder materialincluding 87.0 wt % of Sn and 13.0 wt % of Sb), which is ordinarily usedas a conventional Sn—Sb-based solder material, is of about 300° C. Whenthe semiconductor device is operated in an environment of about 175° C.,in some cases even higher reliability may be necessary, depending on,for instance, the intended application of the device, even forSn—Sb-based solder materials for which the reliability has been enhancedthrough an increase in the content of Sb to bring the melting point ofthe solder material to about 300° C.

The following problems arise in conventional Sn—Sb-based solder jointlayers due to thermal load in power cycling or the like. In FIG. 8, (b)illustrates the state of a conventional Sn—Sb-based solder joint layerat the time of a power cycling reliability test. In conventionalSn—Sb-based solder joint layers, the crystal grain boundaries between Sncrystal grains 131 are not strengthened, as illustrated in (b) of FIG.8; accordingly, a problem arises in that, when the solder is straineddue to stress, a grain boundary crack 133 forms at crystal grainboundaries between Sn crystal grains 131, and this grain boundary crack133 progresses to the crystal grain boundaries between adjacent Sncrystal grains 131.

Further, reflow thermal treatments of solder pastes are ordinarilyperformed in an oven, in a nitrogen (N₂) atmosphere. Performing thermaltreatment at 300° C. or above is however difficult from the viewpoint ofthe heat resistance of a solder paste (the heat resistance of the resinin the solder paste is about 250° C.) and thus, solder materials havinga melting point of about 300° C. are difficult to use in manufacturingprocesses. Although thermal treatment is possible at 300° C. or above inreflow thermal treatment of solder paste in an oven having a hydrogen(H₂) atmosphere, there is a risk of damage to the semiconductor chip asa result of thermal treatment at a temperature of 350° C. or above.Softening of aluminum (Al) and copper used as electrode materials andstructural materials, as well as shorter life and shape defects, arefurther concerns that arise in a case where the thermal treatment isperformed for a prolonged period of about 30 minutes, at a temperatureof about 300° C.

SUMMARY OF THE INVENTION

To solve the problems associated with the conventional techniques, asemiconductor device and a manufacturing method of a semiconductordevice are proposed such that the semiconductor device can besolder-joined at a low melting point and has a highly reliable solderjoint layer.

In the semiconductor device, constituent parts in one set thereof arejoined by a solder joint layer, the semiconductor device having thefollowing characterizing features. The solder joint layer includes:first crystal sections containing tin and antimony at a ratio of tinatoms:antimony atoms=1:p (0<p≤0.1); and second crystal sections havingat least one among a first portion containing tin and silver at a ratioof tin atoms:silver atoms=1:q (2≤q≤5) and a second portion containingtin and copper at a ratio of tin atoms:copper atoms=1:r (0.4≤r≤4). Theaverage grain size of the second crystal sections is smaller than theaverage grain size of the first crystal sections.

In the semiconductor device above, the solder joint layer has thirdcrystal sections containing tin and antimony at a ratio of tinatoms:antimony atoms=1:s (0.8≤s≤1.6).

In the semiconductor device, the first crystal sections are tin crystalgrains with antimony in solid solution.

In the semiconductor device, the first crystal sections are tin crystalgrains with antimony in solid solution; and the third crystal sectionsare crystal grains resulting from a reaction between the first crystalsections and antimony that is in excess of the solid solution limit inthe first crystal sections.

In the semiconductor device, the second crystal sections precipitate atcrystal grain boundaries between the first crystal sections.

In the semiconductor device, the average grain size of the first portionis 1 μm or less.

In the semiconductor device, the melting point of the solder joint layeris 260° C. or lower.

Furthermore, the manufacturing method of a semiconductor device in whichconstituent parts in one set thereof are joined by a solder joint layer,includes coating one of the constituent parts with a solder paste thatcontains a mixture of an alloy powder containing antimony, and an alloypowder containing no antimony; and solidifying the solder paste by athermal treatment to form the solder joint layer and join theconstituent parts to each other by way of the solder joint layer. Thesolder joint layer contains first crystal sections and second crystalsections. The first crystal sections contain tin and antimony at a ratioof tin atoms:antimony atoms=1:p (0<p≤0.1). The second crystal sectionshave at least one among a first portion containing tin and silver at aratio of tin atoms:silver atoms=1:q (2≤q≤5) and a second portioncontaining tin and copper at a ratio of tin atoms:copper atoms=1:r(0.4≤r≤4). The average grain size of the second crystal sections issmaller than the average grain size of the first crystal sections.

Other objects, features, and advantages of the present invention arespecifically set forth in or will become apparent from the followingdetailed description of the invention when read in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional diagram illustrating a structure of asemiconductor device according to an embodiment;

FIG. 2 is an explanatory diagram illustrating schematically aconfiguration of a solder joint layer in FIG. 1;

FIG. 3A is a cross-sectional diagram illustrating a state of a solderjoint layer in Example 1 at the time of a power cycling reliabilitytest;

FIG. 3B is a cross-sectional diagram illustrating a state of a solderjoint layer in Example 3 at the time of a power cycling reliabilitytest;

FIG. 4A is a cross-sectional diagram illustrating a state of a solderjoint layer in Comparative Example 1 at the time of a power cyclingreliability test;

FIG. 4B is a cross-sectional diagram illustrating a state of a solderjoint layer in Comparative Example 2 at the time of a power cyclingreliability test;

FIG. 5 is a characteristic diagram illustrating a relationship betweenSb content in a semiconductor device and power cycling reliabilitytolerance;

FIG. 6 is a characteristic diagram illustrating a relationship betweenAg content in a semiconductor device and power cycling reliabilitytolerance;

FIG. 7 is an explanatory diagram illustrating schematically a state of asolder joint layer by a conventional Sn—Ag-based solder material;

FIG. 8 is an explanatory diagram illustrating schematically a state of asolder joint layer by a conventional Sn—Sb-based solder material;

FIG. 9 is an explanatory diagram illustrating schematically a state, atthe time of melting, of a homogeneous paste for forming the solder jointlayer in FIG. 1;

FIG. 10 is an explanatory diagram illustrating schematically a state, atthe time of melting, of a mixed paste for forming the solder jointlayers of FIG. 1;

FIG. 11 is a cross-sectional diagram illustrating a state of a solderjoint layer in Example 4 at the time of a power cycling reliabilitytest;

FIG. 12 is a cross-sectional diagram illustrating a state of a solderjoint layer in Example 5 at the time of a power cycling reliabilitytest;

FIG. 13 is a cross-sectional diagram illustrating another state of asolder joint layer in Example 4 at the time of a power cyclingreliability test;

FIG. 14 is a cross-sectional diagram illustrating another state of asolder joint layer in Example 5 at the time of a power cyclingreliability test; and

FIG. 15 is a cross-sectional diagram illustrating another state of asolder joint layer in Example 1 at the time of a power cyclingreliability test.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of a semiconductor device and a manufacturingmethod of a semiconductor device according to the present invention willbe described in detail with reference to accompanying drawings. In thedescription of the embodiments below and accompanying drawings,identical structures are denoted by identical reference symbols, and arecurrent description thereof will be omitted.

A structure of a semiconductor device according to an embodiment will bedescribed. FIG. 1 is a cross-sectional diagram illustrating a structureof a semiconductor device according to an embodiment. The semiconductordevice according to the embodiment is, for instance, a modular-structuresemiconductor device that includes a semiconductor chip 1, an insulatingsubstrate 2 such as ceramic insulating substrate (Direct Copper Bonding(DCB) substrate), and a copper (Cu) base 6, as illustrated in FIG. 1. Aheat sink, a resin case, outer terminals, bonding wires and so forthhave been omitted in FIG. 1. The insulating substrate 2 is providedwith, for instance, a circuit pattern (metal foil) 4 including aconductor such as Cu, on the front face side of an insulating layer 3,and is provided with a metal foil, such as a back copper foil 5, on therear face side.

The rear face of the semiconductor chip 1 and the circuit pattern (metalfoil) 4 are joined via a solder joint layer 11. The front face of the Cubase 6 is joined to the back copper foil 5 via a solder joint layer 12.Although not illustrated in the figure, a heat sink is joined to therear face of the Cu base 6 via a thermal compound. A resin case providedwith external terminals is bonded to the peripheral edge of the Cu base6. An electrode (not shown) provided on the front face of thesemiconductor chip 1 and the circuit pattern (metal foil) 4 areelectrically connected to each other by wire bonding, for instance byway of aluminum wires not shown.

A method of joining members to be joined by the solder joint layers 11,12 involves bringing the members to be joined into contact with eachother, via a solder paste or the like, and thereafter, performing athermal treatment, while holding the members to be joined for about 0.5minutes to 30 minutes, preferably for 1 minute to 5 minutes, at atemperature ranging from about 250° C. to 350° C. Thereafter, cooling ata predetermined temperature decrease rate solidifies the solder pasteand forms the solder joint layers. The temperature rise rate in thethermal treatment is about 1° C./sec. Preferably the temperaturedecrease rate is about 5° C./sec or greater, and ranges more preferablyfrom 8° C./sec to 15° C./sec. In conventional joining methods, thetemperature decrease rate in a thermal treatment for forming solderjoint layers has been of 1° C./sec, but solder joint layers having apredetermined configuration failed to be obtained and power cyclingreliability was poor due to the occurrence of cracks in the solder jointlayers.

In the present invention, by contrast, the solder joint layers 11, 12having a metal structure as described below can be obtained byprescribing the above condition of temperature decrease rate.

The oven environment herein may be a nitrogen atmosphere, a hydrogenatmosphere, etc. The above members to be joined are constituent parts ofthe semiconductor device, such as the semiconductor chip, the circuitpattern (metal foil) 4, the metal foil (insulating substrate) and theheat spreader (Cu base) and so forth. The constituent parts that arejoined are, specifically, the semiconductor chip 1, the circuit pattern(metal foil) 4, the Cu base 6, the back copper foil 5, a lead frame, themetal foil (insulating substrate) and so forth.

The solder joint layers 11, 12 are formed using a cream-like solderpaste resulting from mixing a powder of a solder material that includespredetermined materials, at predetermined proportions, and a flux (rosinor the like). It suffices that the solder paste for forming the solderjoint layers 11, 12 has appropriate viscosity such that the solder pastewets, and spreads over, a predetermined surface area, and such that thesolder paste can be applied onto members to be joined using a dispenseror the like. The solder paste may be a solder paste (hereafter referredto as homogeneous paste) that includes a powder of one type of alloy, ormay be a solder paste (hereafter referred to as mixed paste) thatincludes powders of two or more types of alloy that are adjusted todifferent compositions. To join members to be joined to each other usingthe solder paste, for instance the solder paste that yields the solderjoint layers 11, 12 is applied onto one of the members to be joined.Thereafter, other members to be joined are disposed on the solder paste,and the solder joint layers 11, 12 are formed through solidification ofthe solder paste as a result of a thermal treatment. The members to bejoined become bonded to and integrated with each other as a result. Amaterial ordinarily used in semiconductors or the like may be usedherein as the flux included in the solder paste.

A powder adjusted to a predetermined composition may be used as thepowder of the solder material included in the solder paste for formingthe solder joint layers 11, 12. For instance, a powder of an 89Sn8Sb3Agalloy (including 89.0 wt % of Sn, 8.0 wt % of Sb, and 3.0 wt % of Ag)may be used in a case where, for instance, the solder joint layers 11,12 are formed using the 89Sn8Sb3Ag solder material (i.e. formation of ahomogeneous paste). As the powder of the solder material included in thesolder paste for forming the solder joint layers 11, 12, a mixture ofpowders of two or more alloy types adjusted to different compositions(i.e., formation of a mixed paste) may be used. For a powder mixture oftwo or more types of alloy, a solder paste may be used resulting frommixing a first powder including Sb and a second powder including no Sb,at a predetermined weight ratio. Specifically, for instance, a firstpowder of an 81.5Sn16Sb2.5Ag alloy (alloy including 81.5 wt % of Sn,16.0 wt % of Sb and 2.5 wt % of Ag), and a second powder of a96.5Sn3.5Ag alloy (alloy including 96.5 wt % of Sn and 3.5 wt % of Ag),at a weight ratio of 1:1 are mixed to yield a solder paste. The solderjoint layers 11, 12 of the 89Sn8Sb3Ag alloy can be formed by subjectingthis solder paste to a thermal treatment.

The configuration of the solder joint layers 11, 12 will be described indetail. FIG. 2 is an explanatory diagram illustrating schematically aconfiguration of the solder joint layer in FIG. 1. In FIG. 2, (a)illustrates an initial state (before application of a thermal load frompower cycling) of the solder joint layers 11, 12. The solder jointlayers 11, 12 are formed in by a common solder joining method using asolder material that includes predetermined amounts of tin (Sn),antimony (Sb), and silver (Ag). The solder joint layers 11, 12 mayfurther contain Cu at a predetermined proportion. In this case, thesolder joint layers 11, 12 may be formed using a solder material thatincludes predetermined amounts of Sn, Sb, Ag and Cu. Solder wettabilitycan be enhanced by the inclusion of Ag in the solder joint layers 11,12.

As illustrated in (a) of FIG. 2, the solder joint layers 11, 12 have astructure in which plural second crystal sections (crystal grains) 22having, for instance, a fine-grained or columnar shape, precipitate atcrystal grain boundaries between the first crystal sections 21 that aredispersed as a matrix, such that the second crystal sections 22 areharder and have a smaller grain size (diameter) than the first crystalsections 21. The first crystal sections 21 are Sn crystal grains thatinclude Sn and Sb, with the amount of Sb atoms with respect to Sn atomsbeing greater than 0 up to the solid solution limit, for instance aratio of Sn atoms:Sb atoms=1:p (0<p≤0.1), such that the crystal grainsoverall are solid-solution strengthened by Sb that forms a solidsolution in the first crystal sections 21, and the crystals of the firstcrystal sections 21 do not deform readily.

The crystal grain boundaries between the first crystal sections 21 arestrengthened, and crystals do not deform readily, through precipitationof the plural second crystal sections 22 at crystal grain boundariesbetween the first crystal sections 21. The ratio of Sn atoms:Sb atoms isthe ratio of the number of atoms of Sn and Sb. From the viewpoint ofreliability, the average grain size of the first crystal sections 21preferably ranges from 0.2 μm to 100 μm. The reason for this is that agrain size close to 0.2 μm for the first crystal sections 21 affordsresistance against thermal load, whereas, when the average grain sizeexceeds 100 μm, voids occur and for instance, thermal properties andmechanical properties become uneven, as a result of which reliabilitymay decrease. A further reason is that the second crystal sections 22are formed readily at the crystal grain boundaries between the firstcrystal sections 21 in a case where the average grain size of the firstcrystal sections 21 lies within the above range.

The second crystal sections 22 are a first intermetallic compound (firstportion) 22-1 that includes Sn and Ag at a ratio for instance of Snatoms:Ag atoms=1:q (2≤q≤5). Among plural first intermetallic compounds22-1, the average grain size of most of the first intermetalliccompounds 22-1 is preferably 10 μm or smaller, and ranges preferablyfrom 0.1 μm to 1.0 μm, from the viewpoint of reliability. A greatercontent of the first intermetallic compound 22-1 having a grain size of1 μm or smaller in the solder joint layers 11, 12 is preferable, since,accordingly, the strengthening mechanism at the crystal grain boundariesbetween the first crystal sections 21 becomes more pronounced.

The Ag content in the second crystal sections 22 varies depending on,for instance, the content of Sb in the solder material, and the presenceof other atoms during solder joining. The first intermetallic compounds22-1 are for instance an Ag₃Sn (Sn atoms:Ag atoms=1:3) compound or anAg₄Sn (Sn atoms:Ag atoms=1:4) compound including Ag and Sn.

The second crystal sections 22 are present at crystal grain boundariesbetween the first crystal sections 21, and have a portion at which thefirst crystal sections 21 are connected to each other via the secondcrystal sections 22. A portion may be present at which the first crystalsections 21 form a direct interface with each other. The second crystalsections 22 may be formed between the member to be joined and the firstcrystal sections 21, or may be formed between the first crystal sections21 and other second crystal sections 22 of different composition.Further, the second crystal sections 22 may be formed between thirdcrystal sections 23 described below, or between the third crystalsections 23 and the first crystal sections 21, or between the thirdcrystal sections 23 and the member to be joined. Through formation ofthe second crystal sections 22, cracks occur less readily at crystalgrain boundaries of the first crystal sections 21, indicating that thecrystal grain boundaries of the first crystal sections 21 arestrengthened.

The proportion of the surface area of the first intermetallic compound22-1 with respect to the surface area of the first crystal sections 21(hereafter referred to as surface area ratio S1 of the firstintermetallic compound 22-1) may be for instance greater than 0%, up to5% (0%<S1≤5%). An effect of preventing progress of grain boundary cracksis obtained by setting the surface area ratio S1 of the firstintermetallic compound 22-1 to be greater than 0%. Preferably, thesurface area ratio S1 of the first intermetallic compound 22-1 mayrange, for instance, from 2% to 5% (2%≤S1≤5%). The reasons for this areas follows. By prescribing the surface area ratio S1 of the firstintermetallic compound 22-1 to be equal to or greater than 2%, the firstcrystal sections 21 can be covered, substantially completely, by thefirst intermetallic compound 22-1, and accordingly, it becomes possibleto enhance the effect of preventing the progress of grain boundarycracks. A further reason is that when the surface area ratio S1 of thefirst intermetallic compound 22-1 is greater than 5%, the grain size ofthe first intermetallic compound 22-1 (for instance, the Ag₃Sn compound)increases and as a result, the effect of preventing the progress ofgrain boundary cracks is weakened. The surface area ratio and theaverage grain size are calculated through image processing, over aregion sufficiently larger than the grain size of the first crystalsections 21 (for instance a 30 μm×30 μm region) of, for instance,Scanning Electron Microscope (SEM) images at a magnification of 1500×,which allows discriminating a grain size of about 1 μm of the firstintermetallic compound 22-1. More specifically, the outline of the grainis made distinct by image processing, to recognize predetermined grains.The surface area and grain size of the grains are then obtained throughapproximation to circles, polygons or the like.

The second crystal sections 22 may be a second intermetallic compound(second portion) 22-2 that includes Sn and Cu at a ratio of, forinstance, Sn atoms:Cu atoms=1:r (0.4≤r≤4). The second crystal sections22 may be made up of the first intermetallic compound 22-1 and thesecond intermetallic compound 22-2. The second intermetallic compound22-2 is for instance a Cu Sn₅ (Sn atoms:Cu atoms=5:6) compound or aCu₃Sn (Sn atoms:Cu atoms=1:3) compound. The Cu₃Sn compound in the secondcrystal sections 22 is formed through reaction between Cu that is moltenin the solder joint layers 11, 12, from a Cu member (circuit pattern 4or back copper foil 5), and Sn (for instance, first crystal sections21). The Cu₃Sn compound is generated for instance through thermaltreatment over a reaction time lasting 0.5 minutes to 30 minutes,preferably 1 minutes to 5 minutes, at a temperature ranging from 250° C.to 350° C. The Cu₃Sn compound is present in the vicinity of the Cumember, within the solder joint layers 11, 12. The temperature decreaserate in the thermal treatment at the time where the Cu₃Sn compound isgenerated is preferably 5° C./sec or higher, and more preferably rangesfrom 8° C./sec to less than 15° C./sec.

In some instances, the Cu₃Sn compound is formed not only in the vicinityof the Cu member but throughout the solder joint layers 11, 12, bydiffusion of Cu, depending on the thermal load from power cycling(change of temperature in one cycle from room temperature (e.g., 25° C.)up to 175° C.). The second intermetallic compound 22-2 is likewisegenerated by thermal load from power cycling in which the temperature inone cycle changes from room temperature to a temperature in the range of150° C. to 250° C. The underlying reasons are deemed to be as follows.The temperature decrease rate of a semiconductor device upon repeatedswitching on and off lies in a range of 5° C./sec to 10° C./sec. It isdeemed that, as a result, the holding temperature and the rapid coolingconditions in the power cycling test are appropriate for the generationof the second intermetallic compound 22-2 (Cu—Sn compound) which is thesecond portion of the second crystal sections 22.

As generation of the second intermetallic compound 22-2 progresses, theSb concentration in the first crystal sections 21 rises due to theconsumption of Sn in the first crystal sections 21. As a result, thefirst crystal sections 21 are strengthened to a greater degree than in acase of simple solder, and the third crystal sections 23 furtherdescribed hereinafter are newly generated (if already present, then thenumber of third crystal sections 23 increases), all of which elicits areliability enhancement effect. An effect similar to that elicited whenusing Cu is achieved herein by using, besides Cu, also another materialthat forms compounds with Sn, for instance nickel (Ni), gold (Au) andAg, as the surface of members to be joined by the solder joint layers11, 12. Preferably, the average grain size of the second intermetalliccompound 22-2 is 10 μm or smaller, and ranges preferably from 0.1 μm to1.0 μm, from the viewpoint of reliability.

Accordingly, after solder joining by the solder joint layers 11, 12, itis preferable to generate the second intermetallic compound 22-2 byperforming a thermal treatment before actual use. The thermal load atthe time of this thermal treatment involves one or more repetitions,every several seconds to every several minutes, of one cycle in whichthe temperature changes from room temperature to a temperature withinthe range from 150° C. to 250° C. The thermal treatment may involvemaintaining a temperature within the range from 150° C. to 250° C., forseveral minutes. Preferably, the temperature decrease rate in thisthermal treatment is about 5° C./sec or greater, and ranges morepreferably from 8° C./sec to less than 15° C./sec. A temperaturedecrease rate of 15° C./sec or greater is undesirable in the thermaltreatment, since it gives rise to thermal stress in other members, andbetween members. Air cooling or a coolant may be used to perform such arapid cooling process.

The proportion of the surface area of the second intermetallic compound22-2 with respect to the surface area of the first crystal sections 21(hereafter referred to as surface area ratio S2 of the secondintermetallic compound 22-2) may be, for instance, greater than 0% up to50% (0%<S2≤50%). The reasons for this are as follows. The greater thesurface area ratio S2 of the second intermetallic compound 22-2, themore pronounced is the effect that can be achieved of preventing theprogress of the grain boundary cracks. A further reason is that when thesurface area ratio S2 of the second intermetallic compound 22-2 isgreater than 50%, solderability is impaired because the secondintermetallic compound 22-2 becomes a hindrance and voids (bubbles) inthe solder that melts during solder joining can no longer escapereadily.

Therefore, the surface area ratio of the first crystal sections 21 andthe second crystal sections 22 is preferably greater than 2% up to 55%.

The solder joint layers 11, 12 may have the third crystal sections 23that result from a reaction between the first crystal sections 21 and Sbin excess of the solid solution limit, in the first crystal sections 21.The third crystal sections 23 are crystal grains that include Sn and Sb,for instance, at a ratio of Sn atoms:Sb atoms=1:s (0.8≤s≤1.6). Morespecifically, the third crystal sections 23 are an intermetalliccompound, such as a SnSb (Sn atoms:Sb atoms=1:1) compound or a Sb₂Sn₃(Sn atoms:Sb atoms=3:2) compound, harder than the first crystal sections21. The solid solution limit (saturating amount) of Sb with respect tothe Sn crystal grains varies depending on, for instance, the thermaltreatment temperature and the cooling temperature at the time of solderjoining, the content of Sb in the solder material, and the presence ofother atoms at the time of solder joining. Preferably, the average grainsize of the third crystal sections 23 ranges from 0.1 μm to 100 μm, fromthe viewpoint of reliability. This can be ascribed to the same reasonsas in the case of the first crystal sections 21. A grain size of thethird crystal sections 23 larger than 100 μm is undesirable, since, inthat case, solderability is impaired because the third crystal sections23 become a hindrance and voids in the solder that melts during solderjoining can no longer escape readily.

The proportion of the surface area of the third intermetallic compound23 with respect to the surface area of the first crystal sections 21(hereafter referred to as surface area ratio S3 of the thirdintermetallic compound 23) may be for instance greater than 0% up to 15%(0%<S3≤15%). The reasons for this are as follows. The greater thesurface area ratio S3 of the third intermetallic compound 23, the morepronounced is the effect that can be achieved of preventing the progressof the grain boundary cracks. A surface area ratio S3 of the thirdcrystal sections 23 greater than 15% is undesirable, since in that casesolderability is impaired because the third crystal sections 23 become ahindrance and voids in the solder that melts during solder joining canno longer escape readily. Good compositions have been revealed incomposition analysis by Energy-Dispersive X-Ray Spectrometry (EDX),Auger Electron Spectroscopy (AES) and the like, performed on pluralcross-sections of actual first crystal sections 21, second crystalsections 22 and third crystal sections 23.

In FIG. 2, (b) illustrates the state of the solder joint layers 11, 12having such a configuration at the time of a power cycling reliabilitytest (state resulting from being subject to a thermal load from powercycling).

The power cycling reliability test was performed through repeatedenergization with a current-on time of 0.5 seconds to 3 seconds and acurrent-off time of 0.5 seconds to 20 seconds, under a condition wherethe temperature during one cycle varied from room temperature up to 175°C. consequent to heat generation (test time: 50 hours).

As described above, the entire first crystal sections 21 undergo solidsolution strengthening by the Sb in solid solution and, accordingly, thefirst crystal sections 21 exhibit no coarsening even when subject to athermal load from power cycling. Therefore, the precipitationstrengthening mechanism at crystal grain boundaries between the firstcrystal sections 21, elicited by the second crystal sections 22, doesnot collapse. Even if a grain boundary crack or intra-grain crack(hereafter referred to as crack 24) is generated at one first crystalsection 21, as illustrated in (b) of FIG. 2, it becomes accordinglypossible to reduce the progress of the crack 24 in the first crystalsections 21 that are contiguous to the first crystal section 21 in whichthe crack 24 has occurred, as well as the progress of the crack 24 atcrystal grain boundaries between the first crystal sections 21. Themelting mechanism of the solder paste for forming the solder jointlayers 11, 12 will be described. The melting mechanism of a homogeneouspaste will be described first. FIG. 9 is an explanatory diagramillustrating schematically a state, at the time of melting, of ahomogeneous paste for forming the solder joint layer in

FIG. 1. In FIG. 9, (a) illustrates the state of the homogeneous pastebefore a thermal treatment, and (b) in FIG. 9 illustrates the state ofthe homogeneous paste during a thermal treatment. As illustrated in (a)of FIG. 9, the homogeneous paste before the thermal treatment has aconfiguration where, for instance, plural Ag₃Sn compounds 62 having forinstance a fine-grained to columnar shape and having a grain size(diameter) smaller than that of 92Sn8Sb crystal grains 61 (crystalgrains including 92.0 wt % of Sn and 8.0 wt % of Sb) that are dispersedin a matrix, are precipitated at crystal grain boundaries between the92Sn8Sb crystal grains 61. The homogeneous paste includes only one typeof alloy powder and, accordingly, the structure made up of theabove-described 92Sn8Sb crystal grains 61 and the Ag₃Sn compounds 62 isdistributed homogeneously throughout the homogeneous paste.

When the homogeneous paste is subjected to a thermal treatment forforming the solder joint layers 11, 12, portions of low Sbconcentration, i.e. the Ag₃Sn compounds 62 that include no Sb of highmelting point, are the first to melt, as illustrated in (b) of FIG. 9,when the temperature of the thermal treatment reaches for instance ofabout (221+α)° C. The Ag₃Sn compounds 62 are present in a state of beingpartly dispersed throughout the homogeneous paste; accordingly, theapparent melting point of the homogeneous paste as a whole exhibitsvirtually no change with respect to the melting point of the 92Sn8Sbcrystal grains 61, even though the Ag₃Sn compounds 62 melt sooner thanthe 92Sn8Sb crystal grains 61. In the case of a homogeneous paste,therefore, the temperature of the thermal treatment reaches the meltingpoint of the Ag₃Sn compounds 62, whereupon the Ag₃Sn compounds 62 melt.Thereafter overall liquefaction occurs when the temperature reaches themelting point of the 92Sn8Sb crystal grains 61. The occurrence of voidsin the homogeneous paste is a concern if the thermal treatment time isshort. Prolonging the thermal treatment time favors wetting andspreading of the homogeneous paste. Accordingly, it is preferable to seta thermal treatment time such that no voids occur. For instance,virtually no voids are observed to occur in the solder joint layers 11,12 that utilize the homogeneous paste in a case where the thermaltreatment is performed at an oven temperature of 260° C. (235° C. as thetemperature of a heating plate on which a semiconductor chip is placed),in a nitrogen atmosphere, for about 270 seconds or longer. The meltingmechanism of a mixed paste will be described. FIG. 10 is an explanatorydiagram illustrating schematically a state, at the time of melting, of amixed paste for forming the solder joint layers in FIG. 1. In FIG. 10,(a) illustrates the state of the mixed paste before a thermal treatment,and (b) in FIG. 10 illustrates the state of the mixed paste during athermal treatment. As illustrated in (a) of FIG. 10, the mixed pasteincludes a first powder 70-1 and a second powder 70-2, in a separatedstate, at a predetermined weight ratio. The first powder 70-1 is apowder including Sb and has, for instance, a structure in which Sncrystal grains 71-1 with Sb in solid solution are dispersed in a matrix.Reference symbol 71-2 denotes a SnSb compound in which Sb in excess ofthe solid solution limit precipitates along with part of the Sn in theSn crystal grains 71-1. The second powder 70-2 is a powder including noSb and having, for instance, a structure in which plural Ag₃Sn compounds72-2 having a fine-grained to columnar shape and having a grain sizesmaller than that of Sn crystal grains 72-1 dispersed in a matrix, areprecipitated at crystal grain boundaries between the Sn crystal grains72-1.

When the mixed paste is subjected to a thermal treatment for forming thesolder joint layers 11, 12, portions of low Sb concentration, i.e., thesecond powder 70-2 that includes no Sb of high melting point, are thefirst to melt and liquefy, as illustrated in (b) of FIG. 10, when thetemperature of the thermal treatment reaches for instance about 221° C.That is, the entirety of the second powder 70-2 liquefies, whereuponpart of the mixed paste is brought to a liquefied state. The secondpowder 70-2 that has melted first diffuses (not shown in the figures)into the first powder 70-1, and the mixed paste as a whole liquefies ina shorter time than the homogeneous paste does. Thus, the apparentmelting point of the mixed paste as a whole drops consequent to thesecond powder 70-2, which has a lower melting point since it includes noSb. The wettability of the mixed paste is enhanced through liquefactionin a shorter time than in the case where a homogeneous paste is used. Asa result, the occurrence of voids can be suppressed to a greater degreethan is the case when a homogeneous paste is used. For instance, voidsoccur substantially throughout the solder joint layers 11, 12 thatutilize a homogeneous paste in a case where the thermal treatment isperformed at an oven temperature of 260° C. (235° C. as the temperatureof a heating plate on which a semiconductor chip is placed), in anitrogen atmosphere, for about 110 seconds. By contrast, virtually novoids are observed to occur in solder joint layers 11, 12 that utilize amixed paste.

The solder joint layers 11, 12 described above were checked on the basisof a power cycling reliability test. An explanation follows first on theresults of the power cycling reliability test in a case where the solderjoint layers 11, 12 were formed using a homogeneous paste. For instance,solder joint layers 11, 12 having a thickness of 100 μm were formed bysolder joining using a homogeneous paste including a 89Sn8Sb3Ag soldermaterial (solder material including 89.0 wt % of Sn, 8.0 wt % of Sb and3.0 wt % of Ag, melting point about 253° C.) (i.e. a homogeneous pasteincluding a powder of 89Sn8Sb3Ag alloy, hereafter referred to as ahomogeneous paste including a solder material), and the state of thesolder joint layers 11, 12 at the time of a power cycling reliabilitytest was observed. The results are illustrated in FIG. 3A (hereafter,Example 1). The temperature in the thermal treatment of Example 1 wasset to 270° C., the hold time to 5 minutes, and the temperature decreaserate to 10° C./sec. Solder joint layers 11, 12 were formed by solderjoining using a homogeneous paste including a 84Sn13Sb3Ag soldermaterial (solder material including 84.0 wt % of Sn, 13.0 wt % of Sb and3.0 wt % of Ag, melting point about 290° C.), and the state of thesolder joint layers 11, 12 at the time of a power cycling reliabilitytest was observed. The results are illustrated in FIG. 3B (hereafter,Example 3). The temperature in the thermal treatment of Example 3 wasset to 320° C., the hold time to 5 minutes, and the temperature decreaserate to 10° C./sec. As Comparative Example 1, a solder joint layer wasformed by solder joining using a homogeneous paste including aconventional 87Sn13Sb solder material (solder material including 87.0 wt% of Sn and 13.0 wt % of Sb, melting point about 300° C.), and the stateof the solder joint layer at the time of a power cycling reliabilitytest was observed. The results are depicted in FIG. 4A. The temperaturein the thermal treatment of Comparative Example 1 was set to 320° C.,the hold time to 5 minutes, and the temperature decrease rate to 10°C./sec. FIG. 3A is a cross-sectional diagram illustrating the state ofthe solder joint layer of Example 1 at the time of the power cyclingreliability test. FIG. 3B is a cross-sectional diagram illustrating thestate of the solder joint layer of Example 3 at the time of the powercycling reliability test. FIG. 4A is a cross-sectional diagramillustrating the state of solder joint layer of Comparative Example 1 atthe time of the power cycling reliability test. FIGS. 3A, 3B, 4A are SEMimages observed from the semiconductor chip surface (the same applies toFIGS. 4B, 11, 12).

As illustrated in FIG. 3A, plural Sn crystal grains 31 having Sb insolid solution were observed to be dispersed, as a matrix, in a solderjoint layer 30-1 of Example 1, and precipitation was observed of afine-grained and hard Ag₃Sn compound 32-1 (first intermetallic compoundof second crystal sections) having a grain size equal to or smaller than0.5 μm, so as to surround the Sn crystal grains 31 (average grain sizeabout 30 μm) at crystal grain boundaries between Sn crystal grains 31(first crystal sections). It was found that one or more Sn crystalgrains 31 reacted with Sb in excess of the solid solution limit, toyield a SnSb compound 33 (third crystal sections), while a Cu Sn₅compound 32-2 (second intermetallic compound of the second crystalsections) was observed to precipitate in the vicinity of the joininginterface between the solder joint layer 30-1 and a Cu member 30-2. ThisCu Sn₅ compound 32-2 is formed during solder joining (for instance,thermal treatment at a temperature of 270° C. for about 5 minutes). As aresult, the first intermetallic compound and the second intermetalliccompound of the second crystal sections are formed so as to surround oneor plural first crystal sections. Part of the second crystal sectionsare formed as a result of having been subject to a thermal load frompower cycling (change of temperature during one cycle from roomtemperature to 175° C.). It was found that further thermal load frompower cycling did not result in coarsening of the grain size of the Sncrystal grains 31, or in collapse of the precipitation strengtheningmechanism at the crystal grain boundaries between the Sn crystal grains31, as elicited by the Ag₃Sn compound 32-1, the Cu Sn₅ compound 32-2 andthe SnSb compound 33, nor were cracks observed to occur. The secondintermetallic compound of the second crystal sections is likewisegenerated by thermal load from power cycling in which the temperature inone cycle changes from room temperature to a temperature in the range of150° C. to 250° C. As illustrated in FIG. 3B, in Example 3 as well, theSn crystal grains 31, the Ag₃Sn compound 32-1 and the SnSb compound 33were observed to precipitate, as in the case of Example 1. On the otherhand, no second crystal sections were present in the solder joint layer40 of Comparative Example 1 and, accordingly, the occurrence of cracks44 was observed (FIG. 4A) at crystal grain boundaries between Sn crystalgrains 41, as a result of solder strain brought about by thermal stress.

The results of the power cycling reliability test in a case where thesolder joint layers 11, 12 were formed using a mixed paste will bedescribed. Solder joint layers 11, 12 were formed by solder joiningusing a mixed paste, and the state of the solder joint layers 11, 12 atthe time of a power cycling reliability test was observed. The resultsare illustrated in FIGS. 11, 12 (hereafter, Examples 4 and 5). FIG. 11is a cross-sectional diagram illustrating a state of the solder jointlayer of Example 4 at the time of the power cycling reliability test.FIG. 12 is a cross-sectional diagram illustrating a state of the solderjoint layer of Example 5 at the time of the power cycling reliabilitytest. FIGS. 11, 12 illustrate Examples 4 and 5 of a case where thethermal treatment was performed with the thermal treatment temperatureset to 260° C., in a nitrogen atmosphere, and for about 300 seconds (5minutes). FIGS. 13, 14 illustrate respectively Examples 4 and 5 in whichthe thermal treatment was performed with the thermal treatmenttemperature set to 230° C. (maximum 232° C.), in a nitrogen atmosphere,for about 300 seconds (5 minutes). The temperature decrease rate was setto 10° C./sec. FIG. 13 is a cross-sectional diagram illustrating anotherstate of the solder joint layer of Example 4 at the time of the powercycling reliability test. FIG. 14 is a cross-sectional diagramillustrating another state of the solder joint layer of Example 5 at thetime of the power cycling reliability test.

In Example 4, a solder joint layer was formed by solder joining using amixed paste resulting from mixing a first powder of a 70Sn30Sb alloy(alloy including 70.0 wt % of Sn and 30.0 wt % of Sb) and a secondpowder of a 96Sn4Ag alloy (alloy including 96.0 wt % of Sn and 4.0 wt %of Ag). The weight ratio of the first powder and the second powder inExample 4 was 1:2.8. In Example 5, a solder joint layer was formed bysolder joining using a mixed paste resulting from mixing a first powderof a 70Sn30Sb alloy (alloy including 70.0 wt % of Sn and 30.0 wt % ofSb) and a second powder of a 96Sn4Ag alloy (alloy including 96.0 wt % ofSn and 4.0 wt % of Ag). The weight ratio of the first powder and thesecond powder in Example 5 was 1:1. Third crystal sections (SnSbcompound) were observed to form in Example 4 and 5 as well, by X-RayPhotoelectron Spectroscopy (XPS), as in the case of Example 1. In FIGS.11, 12, the first and third crystal sections are denoted collectively bythe reference symbol 81. FIG. 15 illustrates Example 1 in a case wherethe thermal treatment was performed, for comparison, with the thermaltreatment temperature set to 230° C. (maximum 232° C.), in a nitrogenatmosphere, for about 300 seconds (5 minutes). FIG. 15 is across-sectional diagram illustrating another state of the solder jointlayer of Example 1 at the time of the power cycling reliability test.

The results of FIGS. 11, 12 reveal that in Examples 4 and 5 as well,which involved solder joining using a mixed paste, first crystalsections (including third crystal sections) 81 and second crystalsections 82 were observed to form similarly to the case in Example 1(FIG. 3A), which involved solder joining using a homogeneous paste thatincluded a 89Sn8Sb3Ag solder material. More specifically, it was foundthat, as is the case where the solder joint layers 11, 12 are formedusing a homogeneous paste, it is possible to achieve in the solder jointlayers 11, 12, a homogeneous metal structure in which the first crystalsections (including the third crystal sections) 81 and the secondcrystal sections 82 are arrayed substantially regularly, also in a casewhere the solder joint layers 11, 12 are formed using a mixed paste. Itwas found that in Examples 4 and 5, both the first crystal sections(including the third crystal sections) 81 and the second crystalsections 82 were made yet finer, and a yet more homogeneous metalstructure could be achieved than in the case of Example 1. By using themixed paste, thus, it becomes possible to suppress coarsening of thefirst and third crystal sections, caused by aggregation of the thirdcrystal sections, to a yet greater degree than when using thehomogeneous paste. The reasons for this are as follows.

As illustrated in FIGS. 13, 14, melting was observed to occur at about230° C., in Examples 4 and 5 where a mixed paste was used. Asillustrated in FIG. 15, on the other hand, no melting was observed at atemperature of about 230° C. of the heating plate on which asemiconductor chip was placed, in Example 1, where a homogeneous pastewas used. Although not illustrated in the figures, melting is notcomplete, and voids are observed to occur, at a temperature of about260° C. of the heating plate on which a semiconductor chip was placed,in Example 1, where a homogeneous paste was used. It can thus beconcluded that faster melting in Examples 4 and 5, in which a mixedpaste was used, than in Example 1, in which a homogeneous paste wasused, arises from the second powder, which includes no Sb, melting firstand diffusing into the first powder, in the mixed paste. It is deemedthat progress of liquefaction of the mixed paste in a shorter time inExamples 4 and 5 results in ongoing refinement of the first crystalsections (including the third crystal sections) 81 and the secondcrystal sections 82.

The content of Sb in the solder joint layers 11, 12 will be discussed.FIG. 5 is a characteristic diagram illustrating the relationship betweenSb content in a semiconductor device and power cycling reliabilitytolerance. FIG. 5 illustrates results of measurements of power cyclingreliability tolerance in produced samples of solder joint layers 11, 12by the (100-x-y)SnxSbyAg solder materials including Sn (100-x-y)wt %, Sbx wt % and Ag y wt % of Example 1 (solder joint layer by a homogeneouspaste including a 89Sn8Sb3Ag solder material), Comparative Example 1(solder joint layer by a homogeneous paste including a 87Sn13Sb soldermaterial), Example 3 (solder joint layer by a homogeneous pasteincluding a 84Sn13Sb3Ag solder material), and, in addition, alsoproduced samples of Example 2 and Comparative Example 2. In ComparativeExample 2, a solder joint layer was formed by solder joining using ahomogeneous paste including a 97Sn3Ag solder material (solder materialincluding 97.0 wt % of Sn and 3.0 wt % of Ag).

The temperature in the thermal treatment of Comparative Example 2 wasset to 280° C., the hold time to 5 minutes, and the temperature decreaserate to 10° C./sec. In Example 2, a solder joint layer was formed bysolder joining using a homogeneous paste including a 90Sn8Sb2Ag soldermaterial (solder material including 90.0 wt % of Sn, 8.0 wt % of Sb and2.0 wt % of Ag). The temperature in the thermal treatment of Example 2was set to 270° C., the hold time to 5 minutes, and the temperaturedecrease rate to 10° C./sec.

The black square symbol (▪) of a sample including 97 wt % of Sn and 3 wt% of Ag, i.e. Sb content set to 0 wt % and Ag content set to 3 wt %(power cycling reliability tolerance=100%) denotes a conventionalSn—Ag-based solder joint layer (Comparative Example 2). The powercycling reliability tolerance (%) given in the ordinate axis of FIG. 5was calculated taking Comparative Example 2 as a reference. The abscissaaxis in FIG. 5 represents Sb content (wt %).

Reference line 51 in FIG. 5 denotes the proximity of the melting point260° C. of the solder material, such that the further to the left fromthe reference line 51, the lower the melting point is, and the furtherto the right of the reference line 51, the higher the melting point is.FIG. 4B illustrates a cross-sectional diagram illustrating the state ofsolder joint layer of Comparative Example 2 at the time of the powercycling reliability test. FIG. 4B is a cross-sectional diagramillustrating the state of solder joint layer of Comparative Example 2 atthe time of the power cycling reliability test.

Composition analysis results for the compositions of Examples 1 to 3were as follows. The first crystal sections were Sn atoms:Sb atoms=1:p(0<p≤0.1), the first intermetallic compound (first portion) of thesecond crystal sections was, for instance, an Ag₃Sn (Sn atoms:Agatoms=1:3) compound or Ag₄Sn (Sn atoms:Ag atoms=1:4) compound, within arange of Sn atoms:Ag atoms=1:q (2≤q≤5).

The second intermetallic compound (second portion) in the second crystalsections was mainly, for instance, a Cu Sn₅ (Sn atoms:Cu atoms=5:6)compound or Cu₃Sn (Sn atoms:Cu atoms=1:3) compound, within a range Snatoms:Cu atoms=1:r (0.44). The third crystal sections was a SnSb (Snatoms:Sb atoms=1:1) compound or Sb₂Sn₃ (Sn atoms:Sb atoms=3:2) compound,within a range Sn atoms:Sb atoms=1:s (0.8≤s≤1.6). The average grain sizeof the second crystal sections was observed, by cross-sectional SEMmicroscopy, to be smaller than the average grain size of the firstcrystal sections.

The results of FIG. 5 revealed that the power cycling reliabilitytolerance in Examples 1 to 3 can be increased, with respect to that ofComparative Example 2, by increasing the Sb content to be greater than 0wt %. It was observed that the greater the increase in Sb content, themore the power cycling reliability tolerance could be enhanced. Morespecifically, it was observed that a power cycling reliability toleranceabout twice that of Comparative Example 2 was obtained in the vicinityof 250° C. (melting point of the solder materials for producing Examples1 and 2, and that a power cycling reliability tolerance greater thantwice that of Comparative Example 2 was obtained in the vicinity of 290°C. (melting point of the solder material for producing Example 3). AsFIGS. 3A, 3B reveal, no cracks or the like were observed in Examples 1to 3. Therefore, it was found that the semiconductor device according tothe disclosure of the present application is sufficiently compatiblewith semiconductor devices installed in automobiles and semiconductordevices for new energy applications, used in environments at atemperature of about 175° C. and from which high reliability isrequired. In the case of Comparative Example 2, coarsening of the AgSncompound 42 to a grain size of about 5 μm, as well as the occurrence ofcracks 44, were observed in a solder joint layer 40, as illustrated inFIG. 4B, similar to those in a conventional Sn—Ag-based solder materialto which no Sb is added. This is deemed to be the cause of the poorerpower cycling reliability tolerance. In a case where the Sb content wasgreater than 15 wt % (further to the right of the dotted line denoted byreference symbol 52 in FIG. 5), the melting point of the solder materialrose excessively, and impaired solder wettability was observed.Accordingly, the Sb content in the solder joint layers 11, 12 ispreferably larger than 0 wt %, up to 15 wt %.

The content of Ag in the solder joint layers 11, 12 will be described.FIG. 6 is a characteristic diagram illustrating the relationship betweenAg content in a semiconductor device and power cycling reliabilitytolerance. FIG. 6 illustrates results of measurements of power cyclingreliability tolerance in solder joint layers 11, 12 by (100-x-y)SnxSbyAgsolder materials including Sn (100-x-y)wt %, Sb x wt % and Ag y wt %.

The black triangle symbol (▴) of a sample including 97 wt % of Sn and 3wt % of Ag, i.e. Sb content set to 0 wt % and Ag content set to 3 wt %(power cycling reliability tolerance=100%) denotes Comparative Example 2described above. The power cycling reliability tolerance (%) given inthe ordinate axis of FIG. 6 was calculated taking Comparative Example 2as a reference. The abscissa axis in FIG. 6 represents Ag content (wt%). The black square symbol (▪) of a sample including 87 wt % of Sn and13 wt % of Sb, i.e. Sb content set to 13 wt % and Ag content set to 0 wt% (power cycling reliability tolerance=about 150%) denotes aconventional Sn—Sb-based solder joint layer (Comparative Example 1).

The results of FIG. 6 revealed that the power cycling reliabilitytolerance can be increased with respect to that of Comparative Examples1 and 2 by increasing the Sb content to be greater than 0 wt %, and theAg content to be greater than 0 wt %. It was observed that the greaterthe increase in Ag content, the more the power cycling reliabilitytolerance could be enhanced. In a case where the Ag content was greaterthan 3 wt % (further to the right of the dotted line denoted byreference symbol 53), solderability was observed to decrease, andmaterial cost to increase. Accordingly, the Ag content in the solderjoint layers 11, 12 is preferably larger than 0 wt %, up to 3 wt %.

In the embodiment (Example 2), as described above, a solder joint layeris made up of, and strengthened by, first crystal sections (Sn crystalgrains), having Sb in solid solution, and arrayed substantiallyregularly configuring a uniform metal structure, and by plural secondcrystal sections that precipitate at crystal grain boundaries betweenthe first crystal sections that are dispersed as a matrix. The averagegrain size of the second crystal sections was smaller than the averagegrain size of the first crystal sections. The average grain size of thefirst crystal sections was 30 v, and the average grain size of thesecond crystal sections was 0.8 v. More specifically, the entire firstcrystal sections are solid-solution strengthened by the Sb that forms asolid solution in the first crystal sections. As a result, it becomespossible to suppress coarsening of the first crystal sections consequentto the thermal load in, for instance, power cycling. The crystal grainboundaries between the first crystal sections dispersed in the form of amatrix are strengthened, and crystals of the first crystal sections arethus rendered less likely to deform, by the first intermetallic compound(compound including Sn and Ag) being the second crystal sections thatare harder and more fine-grained than the first crystal sections.Accordingly, the progress of intra-grain cracks and grain boundarycracks can be suppressed to a greater degree than in the case ofconventional Sn—Ag-based solder joint layers and Sn—Sb-based solderjoint layers, and power cycling reliability can be likewise enhanced.

In the embodiments (Examples 1 and 3), part of the first crystalsections constitutes third crystal sections through reaction with Sbthat is in excess of the solid solution limit; as a result,stress-derived strain is less likely to occur in the solder jointlayers. Crystals in the first crystal sections deform less readily as aresult. The third crystal sections are harder than the first crystalsections. Accordingly, progress of intra-grain cracks can be suppressedto a yet greater degree. In turn, this allows further enhanced powercycling reliability tolerance. In the above embodiments, the secondcrystal sections may have a second intermetallic compound (compoundincluding Sn and Cu) that is formed during joining of solder with a Cumember, and consequent to thermal load from power cycling. The crystalgrain boundaries between the first crystal sections dispersed as amatrix are further strengthened by further incorporating the secondintermetallic compound (second portion) in addition to the firstintermetallic compound (first portion) as the second crystal sections.The progress of grain boundary cracks can be suppressed as a result to agreater degree than in the case of conventional Sn—Ag-based solder jointlayers and Sn—Sb-based solder joint layers. Power cycling reliabilitytolerance can therefore be further enhanced.

In the embodiments, the melting point of the solder joint layer can beset to a temperature lower than 300° C., for instance a temperature of260° C. or lower (for example, about 230° C.), by forming the first andthe second crystal sections or the third crystal sections to apredetermined grain size and composition, without addition of Ag to ahigh concentration. Through a soldering process at a temperature lowerthan 300° C., it becomes possible thus to achieve a power cyclingreliability tolerance comparable or superior to that of, for instance,conventional Sn—Sb-based solder joint layers, which require a solderingprocess at a temperature of 300° C. or higher. Since the solderingprocess can be performed at a temperature lower than 300° C., thethermal load that acts on the semiconductor device can be reduced, and ahighly reliable semiconductor device can be provided that is lesssusceptible to the adverse effects of thermal load than conventionalsemiconductor devices. Through formation of a solder joint layer using amixed paste obtained by mixing a first powder including Sb and a secondpowder including no Sb, the embodiments allow imparting the solder jointlayer with a uniform metal structure in which first to third crystalsections are arrayed more substantially regularly than in a case wherethe solder joint layer is formed using a homogeneous paste including apowder of one alloy.

The power cycling reliability tolerance was 230 (%) in Example 4 and 240(%) in Example 5. Thus, a uniform metal structure is obtained in whichthe first to third crystal sections are arrayed substantially regularlyand the first and second crystal sections are made finer, so that powercycling reliability can be enhanced as a result.

The present invention is not limited to the embodiments described above,and may accommodate various modifications without departing from thegist of the invention. In a case where, for instance, the semiconductordevice is provided with plural solder joint layers, the solder jointlayers may have identical or different compositions, so long as thecompositions are within the ranges described above.

Thus, according to the disclosure, a solder joint layer is made up of,and strengthened by, first crystal sections (tin crystal grains) havingantimony in solid solution, and arrayed substantially regularlyconfiguring a uniform metal structure, and second crystal sectionsincluding plural first portions (compound including tin and silver) orsecond portions (compound including tin and copper), or both,precipitated at crystal grain boundaries between the first crystalsections. More specifically, the first crystal sections overall aresolid-solution strengthened by the antimony that forms a solid solutionin the first crystal sections. As a result, it becomes possible tosuppress coarsening of the first crystal sections consequent to thermalload in, for instance, power cycling. The crystal grain boundariesbetween the first crystal sections are strengthened by the secondcrystal sections, such that crystals in the first crystal sections donot deform readily. Accordingly, the progress of intra-grain cracks andgrain boundary cracks can be suppressed to a greater degree than in thecase of conventional tin-silver-based solder joint layers andtin-antimony-based solder joint layers.

According to the disclosure, part of the first crystal sectionsconstitutes third crystal sections through reaction with antimony thatis in excess of the solid solution limit; as a result, stress-derivedstrain does not occur readily in the solder joint layers. Crystals inthe first crystal sections deform less readily as a result. Theinvention enables the melting point of the solder joint layer to be setto a temperature lower than 300° C., for instance a temperature of 260°C. or lower, by way of the first and second crystal sections. As aresult, it becomes possible, through a soldering process at atemperature lower than 300° C., to achieve a power cycling reliabilitytolerance comparable or superior to that of, for instance, conventionaltin-antimony-based solder joint layers, which require a solderingprocess at a temperature of 300° C. or higher. Since the solderingprocess can be performed at a temperature lower than 300° C., thethermal load that acts on the semiconductor device can be reduced. Asemiconductor device can thus be provided that is less susceptible tothe adverse effects of thermal load than conventional semiconductordevices. The power cycling reliability tolerance denotes herein thenumber of repeats upon repeated intermittent energization of asemiconductor device until a necessary predetermined characteristic as asemiconductor device can no longer be obtained consequent to repeatedheat generation and the stress associated therewith. Through formationof a solder joint layer using a mixed paste obtained by mixing a firstpowder including antimony and a second powder including no antimony, thedisclosure enables imparting the solder joint layer with a uniform metalstructure in which first to third crystal sections are arrayed moresubstantially regularly than in a case where the solder joint layer isformed using a homogeneous paste including a powder of one alloy.

The semiconductor device and manufacturing method of a semiconductordevice according to the present invention elicit the effects of enablinga semiconductor device and a manufacturing method of a semiconductordevice to be provided such that the semiconductor device can besolder-joined at a low melting point and has a highly reliable solderjoint layer.

The semiconductor device and manufacturing method of a semiconductordevice according to the disclosure are useful in package-structuresemiconductor devices in which various members such as semiconductorchips and circuit patterns are joined by way of solder joint layers.

Although the invention has been described with respect to a specificembodiment for a complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodifications and alternative constructions that may occur to oneskilled in the art which fairly fall within the basic teaching hereinset forth.

What is claimed is:
 1. A semiconductor device in which constituent partsin one set thereof are joined by a solder joint layer, the solder jointlayer comprising: first crystal sections containing tin and antimony ata ratio of tin atoms:antimony atoms=1:p (0<p≤0.1); and second crystalsections having at least one among a first portion containing tin andsilver at a ratio of tin atoms:silver atoms=1:q (2≤q≤5) and a secondportion containing tin and copper at a ratio of tin atoms:copperatoms=1:r (0.4≤r≤4), wherein average grain size of the second crystalsections is smaller than average grain size of the first crystalsections.
 2. The semiconductor device according to claim 1, wherein thesolder joint layer has third crystal sections containing tin andantimony at a ratio of tin atoms:antimony atoms=1:s (0.8≤s≤1.6).
 3. Thesemiconductor device according to claim 2, wherein the first crystalsections are tin crystal grains with antimony in solid solution, and thethird crystal sections are crystal grains resulting from a reactionbetween the first crystal sections and antimony that is in excess of asolid solution limit in the first crystal sections.
 4. The semiconductordevice according to claim 1, wherein the first crystal sections are tincrystal grains with antimony in solid solution.
 5. The semiconductordevice according to claim 1, wherein the second crystal sectionsprecipitate at crystal grain boundaries between the first crystalsections.
 6. The semiconductor device according to claim 1, whereinaverage grain size of the first portion is 1 μm or less.
 7. Thesemiconductor device according to claim 1, wherein the solder jointlayer has a melting point of 260° C. or lower.
 8. A manufacturing methodof a semiconductor device in which constituent parts in one set thereofare joined by a solder joint layer, the manufacturing method comprising:a step of coating one of the constituent parts with a solder paste thatcontains a mixture of an alloy powder containing antimony, and an alloypowder containing no antimony; and a step of solidifying the solderpaste by a thermal treatment to form the solder joint layer, and joiningthe constituent parts to each other by way of the solder joint layer,wherein the solder joint layer includes: first crystal sectionscontaining tin and antimony at a ratio of tin atoms:antimony atoms=1:p(0<p≤0.1); and second crystal sections having at least one among a firstportion containing tin and silver at a ratio of tin atoms:silveratoms=1:q (2≤q≤5) and a second portion containing tin and copper at aratio of tin atoms:copper atoms=1:r (0.4≤r≤4), and average grain size ofthe second crystal sections is smaller than average grain size of thefirst crystal sections.